(1) Field of the Invention
The present invention relates to a filter medium having such an unexpectedly excellent effect that clogging of the filter medium with a filter cake is not caused at all and the filtration can be performed at a substantially uniform flow rate throughout the operation, and the present invention relates also to a process for preparing this filter medium and a filtering method and apparatus using this filter medium.
By the term "filtration" used in the instant specification is meant an operation of separating at least one component from a dispersion, and by the term "filter medium" is meant a member having a function or mechanism of separating at least one component from a dispersion. Further, by the term "filtering apparatus" used in the instant specification is meant an apparatus for separating at least one component from a dispersion by using a filter medium.
(2) Description of the Prior Art
In conventional filter media or filtering methods, as is well-known, a material to be treated by filtration (the term "dispersion" is used to avoid the confusion in the terminology) is contacted with a filter medium having a great number of small holes, and when the dispersion passes through the filter medium, a substance having a size larger than the mesh size of these small holes is left on the surface of the filter medium, but a substance having a size smaller than the mesh size of the small holes and a liquid (often referred to as "dispersion medium") are allowed to pass through these small holes, whereby filtration or separation is accomplished. During this filtering operation, clogging is inevitably caused more or less and sooner or later in the filter medium, and finally, filtration becomes difficult or extreme reduction of the efficiency is caused. In an extreme case, filtration becomes impossible.
A method in which the efficiency is enhanced by compression or suction is not different from the abovementioned basic method in connection with the principle of filtration or separation. According to this method, only the filtration speed can be elevated, and in some case, clogging takes place rapidly.
When a porous material such as an unglazed plate or sponge or a material consisting of roughly, three-dimensionally and irregularly entangled wires is used as a filter medium, the filtration principle is the same as described above, and clogging is similarly caused.
In case of conventional filter media such as a filter paper and a filter cloth, dispersed phases having a larger size are first caught on holes of the filter medium, and with advance of the filtration, dispersed phases having a smaller size are caught on holes of the filter medium, the size of which has been reduced by the already caught dispersed phases. Thus, dispersed phases having a smaller size are gradually caught on the filter medium. As a result, precise filtration becomes possible. This means, however, that clogging already takes place in the filter medium, and therefore, the filtration speed is gradually reduced and finally, it becomes impossible to continue the filtration operation.
It is known that a so-called filter aid can be used for eliminating this disadvantage. However, even if such filter aid is used, the principle of filtration is the same as described above, and only a large quantity of a dispersion is treated by increasing the filtering area. In this case, the quantity of the dispersion that can be treated is in proportion of the amount of the filter aid used, and therefore, the filtration operation becomes expensive. The filter aid to be incorporated in a dispersion medium consists of particles and voids among these particles act as the above-mentioned holes. Accordingly, when such voids are clogged by a dispersed phase and the degree of clogging is appropriate, the intended precise filtration can be attained and this desirable state can be maintained. However, when the size of particles of the dispersed phase is fine, if clogging is advanced so as to cope with this fine particle size, it is impossible to maintain the fine dispersed phase and the filtration efficiency is inevitably reduced rapidly.
There has been proposed a filter medium comprising a net-like structure composed of three-dimensionally and irregularly entangled linear members such as wires and an effluent pipe connected to this net-like structure to cause a filtrate to flow out therefrom.
Also in this filter medium where linear members are merely entangled three-dimensionally and irregularly to form a net-like structure, as in case of conventional filter media, a filter cake composed of a dispersed phase is intruded into voids in the interior of the net-like structure, and when the particle size of the dispersed phase is small, the dispersed phase is allowed to pass through the net-like structure and no filtering effect can be attained.